Surface Engineering of Specialty Steels

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ASM Handbook, Volume 5: Surface Engineering Copyright © 1994 ASM International® C.M. Cotell, J.A. Sprague, and F.A. Smidt, Jr., editors, p 762-775 All rights reserved. DOI: 10.1361/asmhba0001306 www.asminternational.org

J.R. Davis, Davis & Associates

SPECIALTY STEELS encompass a broad P/M materials for structural parts are iron-copper- amount of admixed graphite and the composition range of ferrous alloys noted for their special carbon, iron-nickel-carbon, and iron-carbon. of the sintering atmosphere. processing characteristics (powder metallurgy al- Parts made from these materials respond to heat loys), corrosion resistance (stainless steels), wear treatment with a defined hardenability band. Iron Deburring P/M Parts (Ref 4) resistance and toughness (tool steels), high parts that are low in carbon and high in density strength (maraging steels), or magnetic properties can also be case hardened. Although cleaning and deburring generally are (electrical steels). Each of these material considered different operations, they are often groupsmwith the exception of stainless steels, Designation of Ferrous P/M Materials accomplished simultaneously. Therefore, much which were discussed in the previous article in of the discussion on deburring is applicable to the this Sectionmwill be reviewed below. Additional Ferrous P/M materials are customarily desig- subsequent section of this article on cleaning. information on these materials can be found in nated by the specifications or standards to which The inherent porosity in P/M parts demands Volumes 1 and 2 of the ASM Handbook. they are made, such as those listed in Table 1. special considerations in all secondary opera- Comparable standards are published by ASTM, tions. This is also true for cleaning and deburring; SAE, and MPIF (Metal Powder Industries Fed- the relatively small size and complex shape of the Ferrous Powder Metallurgy Alloys eration). parts also require special procedures and/or pre- The MPIF designations for ferrous P/M mate- cautions that are not required for wrought or cast Powder metallurgy (P/M) in its simplest form rials, described in detail in Ref 3, include a prefix parts. The P/M parts shown in Fig. 1 are typical consists of compressing metal powders in a of one or more letters (the first of which is F to of small, intricate parts that frequently present shaped die to produce green compacts. These are indicate an iron-base material), four numerals, deburring problems. then sintered, or diffusion bonded, at elevated and a suffix. The second letter in the prefix iden- Deburring Methods. Due to the nature of the temperatures in a furnace with a protective at- tifies the principal alloying element (if one is P/M process, burrs typically form on the edges and mosphere. During sintering, the constituents usu- specified); the percentage of the element is indi- surfaces of P/qVI parts. In many simple shapes, de- ally do not melt, and the compacts become sub- cated by the first two digits. The third and fourth bun'mg is almost automaticmthat is, burrs are bro- stantially strengthened by the development of digits indicate the amount of carbon in the com- ken off during handling operations. If parts are bonds between individual particles. pacted and sintered part; the code designation 00 surface hardened or steam treated, subsequent de- For a specific metal powder and sintering con- indicates less than 0.3 %, 05 indicates 0.3 to 0.6%, burring may be unnecessary. However, for intricate dition, increased compact density results in im- and 0.8 indicates 0.6 to 0.9%. The suffix is used parts such as those shown in Fig. 1, separate debur- proved mechanical properties. The density of sin- to indicate the minimum 0.2% yield strength of ring operations generally are required. tered compacts may be increased by re-pressing. as-sintered parts and the minimum ultimate ten- When re-pressing is performed primarily to in- The use of liquid deburring methods is not sile strength of heat-treated materials in units of crease dimensional accuracy rather than density, usually suitable, especially if such liquids are 1000 psi (6.894 MPa). The letters HT designate it is termed sizing. When re-pressing is intended corrosive. Thus, acid pickling is not recom- heat treated. to change the contour of the surface in contact mended, because acid may be entrapped in the with the punches, it is termed coining. For exam- Commercially produced iron-base powders pores, resulting in severe corrosion. Tumbling in ple, a sintered blank could be coined so that the often contain controlled amounts of alloying ele- a wet medium is used frequently as a deburring surface is indented with small slots or letters and ments other than those specified by any of the method, but removal of the liquid from the pores numbers. The re-pressing may be followed by designations listed in Table 1. Manganese and requires extra drying time. Preferred methods of re-sintering, which relieves the stresses due to molybdenum may be added to improve strength deburring include: cold work and may further strengthen the com- and the response to heat treatment. Sulfur may be pact. More detailed information on consolidation added to enhance machinability. Additions of • Rotary tumbling (self or with abrasive) practices for ferrous P/M parts can be found in 0.45 to 0.80% P can improve the toughness of the • Vibratory Ref 1 and 2. part and reduce magnetic hysteresis losses. These • Abrasive blasting By pressing and sintering only, parts are pro- powders are usually identified by the trade name • Centrifugal or high-energy methods duced at 80 to 93% of theoretical density. By of the producer even though the amounts of alloy re-pressing, with or without sintering, the materi- additions are small enough that the designations These methods, which are used for deburring als may be further densified to 85 to 96% of listed in Table 1 could be applied to the powders. and sometimes for cleaning, are described in theoretical density. High-temperature sintering Commercially produced iron-base powders usu- the articles "Mechanical Cleaning Systems" will also produce parts at these high densities. ally contain very little carbon because carbon and "Mass Finishing Methods" in this Volume. The density of pressed parts is limited by the size lowers compressibility and the amount of carbon The discussions that follow are unique to P/M and shape of the compact. The most common in the finished part is readily controlled by the parts. Surface Engineering of Specialty Steels / 763

Table I Compositionsof ferrous P/M structural materials

Designation(a) MPIF composition limits and ranges, % (b) Description MPIF ASTM SAE Ni Cu Fe Mo

P/M iron F-0000 B 783 853, CI 1 0.3 max ... 97.7-100 P/M steel F-0005 B 783 853, CI 2 0.3-0.6 ... 97.4-99.7 P/M steel F-0008 B 783 853, CI 3 0.6-1.0 97.0-99.1

P/M copper iron FC43200 B 783 ... 0.3 max 1.5~319 93.8-98.5 P/M copper steel FC-0205 B 783 0.343.6 1.5-3.9 93.5-98.2 P/M copper steel FC-0208 B 783 8641Gr 1, CI 3 0.6-1.0 1.5-3.9 93.1-97.9 P/M copper steel FC-0505 B 783 0.3-0.6 4.0-6.0 91.4-95.7 P/M copper steel FC43508 B 783 864;Gr 2, Cl 3 0.6-1.0 4.0-6.0 91.0-95.4 P/M copper steel FC-0808 B 783 864, Gr 3, C13 0.6-1.0 6.0-11.0 86.0-93.4 P/M copper steel 864, Gr 4, CI 3 0.6-0.9 18.0-22.0 75.1 min P/M iron-copper FC-i~0 B783 862 0.3 max 9.5-10.5 87.2-90.5

P/M prealloyed steel FL-4205 B 783 ... 0.443.7 0.35-0.45 ... 95.9-98.7 05o-085

P/M prealloyed steel FL-4605 B 783 0.443.7 1.70-2.00 ... 94.5-97.5 0.4043.80 P/M iron-nickel FN43200 B 783 0.3 max 1.0-3.0 2.5 max 92.2-99.0 P/M nickel steel FN-0205 B 783 0.343.6 1.0-3.0 2.5 max 91.9-98.7 P/M nickel steel FN43208 B 783 0.6-0.9 1.0-3.0 2.5 max 91.6-98.4 P/M iron-nickel FN-0400 B 783 0.3 max 3.0-5.5 2.0 max 90.2-97.0 P/M nickel steel FN-0405 B 783 0.343.6 3.0-5.5 2.0 max 89.9-96.7 P/M nickel steel FN-0408 B 783 0.643.9 3.0-5.5 2.0 max 89.6-96.4 P/M iron-nickel FN-0700 ... 0.3 max 6.0-8.0 2.0 max 87.7-94.0 P/M nickel steel FN-0705 ... 0.3-0.6 6.0-8.0 2.0 max 87.4-93.7 P/M nickel steel FN-0708 0.6-0.9 6.0-8.0 2.0 max 87.1-93.4

P/M infiltrated steel FX-1000 B783 0-0.3 .. 8.0-14.9 82.8-92.0

P/M infiltrated steel FX-1005 B 783 0.343.6 .. 8.0-14.9 80.5-91.7

P/M infiltrated steel FX-1008 B 783 0.6-1.0 .. 8.0-14.9 80.1-91.4

P/M infiltrated steel FX-2000 B 783 870 0.3 max .. 15.0-25.0 70.7-85.0

P/M infiltrated steel FX-2005 B 783 ... 0.343.6 .. 15.0-25.0 70.4-84.7

P/M infiltrated steel FX-2008 B 783 872 0.6-1.0 .. 15.0-25.0 70.0-84.4

(a) Designations listed are nearest comparable designations; ranges and limits may vary slightly between comparable designations. (b) MPIF standards require that the total amount of all other elements be less than 2.0%, except in infiltrated steels, for which the total amount of other elements must be less than 4.0%.

Rotary Tumbling. Self-tumbling, tumbling is used less frequently than tumbling or vibrating. Cleaning of P/M Parts (Ref 4) with dry abrasive, and tumbling with abrasive in a The abrasive must be selected carefully. Coarse shot liquid medium are suitable for deburring of P/M or grit tends to peen the surfaces and close the pores. Some of the deburring methods discussed parts. Wet tumbling is not suitable for deburring Also, abrasive blasting can "hammer" bits of abra- above also may be considered as methods of P/M parts because of the difficulty of removing the sive into the workpiece, thus "charging" it. cleaning. Frequently, however, methods such as tumbling liquid from the pores of the parts. Another disadvantage of conventional abrasive tumbling and blasting are considered as prelimi- During self-tumbling, the workpieces are tum- blasting is that, especially for large volumes of nary cleaning operations to be followed by a bled in a revolving barrel. This method provides small workpieces, results are likely to be nonuni- more thorough cleaning, especially if the parts an economical and efficient means of deburring, form. One type of blasting machine, which tum- are to be coated. but is effective only on relatively simple parts. bles and blasts simultaneously, has been used Cleaning Methods. The inherent porosity in For parts such as those shown in Fig. 1, the successfully for deburring of P/M parts. Silica P/M parts imposes restrictions on selection of clean- internal surfaces and recesses are not completely sand or a milder abrasive is suitable and is less ing method. The use of a cleaning solution that is deburred by this method. For more complete de- likely to damage intricate workpieces. As with corrosive to the metal being cleaned is not recom- burring, an abrasive is added. Size of the abrasive other deburring methods, overprocessing must be mended, because even the most thorough washing is important. At least a portion of the added abra- avoided. is not likely to remove all of the fluid, which pre- sive should have a mesh size that is smaller than Centrifugal or High-Energy Meth- sents a corrosion problem. Acid cleaning is there- the smallest hole or recess in the workpiece; oth- ods. Centrifugal finishing combines rotating ac- fore not recommended. erwise, not all surfaces will be reached. tion with high centrifugal force, which results in a Because of porosity, thorough cleaning of P/M Over-tumbling of P/M parts must be avoided, more severe abrading action than can be obtained by parts is more difficult than their wrought counter- because it peens the surfaces and may partially conventional rotary tumbling. This action is ob- parts; P/M parts require more attention than is close pores (not necessarily desirable). Over- tained by revolving several rotating barrels around provided in many conventional cleaning systems. tumbling also may damage gear teeth or other Preferred methods are hot caustic washing, ultra- protrusions by removing too much metal or by the periphery of a large carrier disk. sonic degreasing, and electrolytic alkaline clean- excessive peening. Tumbling cycles should be As a result, the action within one barrel consists based on the minimum time that will provide of a combination of rotating motion and high ing. acceptable deburring. centrifugal forces, which provides pressures up to Ultrasonic Degreasing. Oils, greases, and Vibratory processing is similar to rotary tum- 25 times the weight of the abrasive medium (if other shop soil may be removed by vapor degreas- bling in principle. However, the shaking involved in used) and the workpiece. As the disk rotates in ing techniques such as vapor phase, vapor-spray-va- the vibratory method is faster and provides more one direction, the barrels rotate at a faster speed por, warm liquid-vapor, or boiling liquid-warm uniform results compared to rotary tumbling. As in in the opposite direction. This counter movement liquid-vapor techniques. For most P/M parts, espe- rotary tumbling, care must be taken to prevent over- within the entire mass accomplishes the desired cially if the degree of soiling is severe and/or part tumbling. results in a shorter time compared to other abrad- density is low, the boiling liquid-warm liquid-vapor Abrasive blasting, in which various materials ing processes. An advantage of this process is that process is preferred. This technique should be used are propelled by air or centrifugal force, offers an- it drives the abrasive into relatively inaccessible in conjunction with an ultrasonic transducer, which other method of deburring. For practical reasons, it areas where burr removal may present problems. literally shakes all entrapped contaminants out of 764 / Surface Engineering of Irons and Steels

Density, Ib/in. 3

0.217 0.224 0.231 0.238 0.246 0.253 0.260 875 127

0.80% C, steam-treated j, 750 o 109 ,m

0.80% C, as-sintered e- e- 625 f 91 l_

t.._ 0.25% C, steam-treated

¢'1

i... / 500 73 =__

0.25% C, as-sintered

~" 375 54

250 36 iiiiiiiiiii!iiiii~iiii ..... ~"i ~iiiiii!iiiiiiii!iiiiiiiiii!iiiiiiiiiiiiii!!iiiiiiiiiiii~iiill~'~...... ~...... ~i!iiiiiii!iiiiiiiii!ilii 6.0 6.2 6.4 6.6 6.8 7.0 7.2 i!iiiiiiiiiii!iiii;'~~ ~iiiiiiiiiiiiiiiili!iiiiiiiiiiiiiiiiiili!iiiiiiiiii...... 'r~iiiiiiiiiii! Density, g/cm 3 (a)

Density, Ib/in. 3 0.217 0.224 0.231 0.238 0.246 0.253 0.260 130

0.80% C, steam-treated iii! i!ii!!iiiiii ii!iiii!!!iiiiiiiiil 110 G'--"- ..-O------

IXI n" "1" 0.25% C, steam-treated 90 Typical shapes of P/M parts that present deburring r ~ Fig. 1 e- 0.80% C, as-sintered or cleaning difficulties t_

r- the pores, resulting in a thorough and safe method 70 A of cleaning. ex Electrolytic Alkaline Cleaning. Ferrous P/M 0.25% C, as-sintered parts can be cleaned, deoxidized, and stripped of nonmetallic coatings by subjecting them to elec- 50 trolysis in a strongly alkaline aqueous solution. In this method the base metal is not attacked, and the possibility of rusting is minimal. Typical alkaline 30 solutions are comprised of a: 6.0 6.2 6.4 6.6 6.8 7.0 7.2 Density, g/cm 3 • Source of caustic to aid cleaning Ib) • Chelating agent to detach scale or rust • Complexing agent to hold relatively large I:.'o, 2 Effectof steam treating on the mechanical properties of sintered carbon P/M steels as a function of density. (a) Trans- II II1~ amounts of iron in solution verse rupture strength. (b) Apparent hardness. Source: Ref 5

Electrolytic alkaline cleaning bath compositions and operating conditions are described in steam atmosphere at temperatures between 510 to filling the porosity with a second phase. Magnet- the article "Alkaline Cleaning" in this Volume. 595 °C (950 to 1100 °F) to form a layer of black ite has a microhardness equivalent to HRC 50. This cleaning process is well suited to cleaning of iron oxide (magnetite, or ferrous-ferric oxide, The process itself is straightforward, the pri- P/M parts, because the electrolytic action pro- FeO-Fe203) in the surface porosity according to mary variables being temPerature, time, and vides additional energy required to dislodge con- the chemical reaction" steam pressure. Caution must be used to prevent taminants from pores or from relatively inacces- the formation of hydroxides and lower oxides sible areas. such as ferrous oxide (FeO) and ferric oxide 3Fe + 4H20 (steam) ~ Fe304 + 4H 2 (gas) (Eq 1) (Fe203), which is red rust. Steam Treating of P/M Parts (Ref 5) The recommended procedure for steam treat- Ferrous P/M parts have traditionally been Steam treating cannot be described as a heat ing is: steam treated for improved wear resistance, cor- treatment because no structural changes occur in rosion resistance, and sealing capacity. Here, P/M the matrix. In this process, magnetite (Fe304) is • Preclean parts to remove any oil or lubricants parts are heated in a specific manner under a formed at the interconnecting surface porosity, that may have been absorbed into the porosity Surface Engineering of Specialty Steels / 765

from prior machining, sizing, or finishing op- Regardless of the method used for coating, Advantages of this painting procedure include: erations major emphasis must be placed on initial clean- • Load clean sintered parts in loosely packed ing. If liquid contaminants are allowed to remain • Precision painting can be achieved without baskets and place fixture into a furnace pre- in the pores of parts, bleeding occurs, and defec- applying paint to areas that do not require a heated to 315 °C (600 °F) tive coatings result. Steam treatment provides an coating • Heat parts in air until the center of the load has excellent paint base. • With proper design of parts, areas can be stabilized at the set temperature Mechanical coating uses kinetic energy to painted with sharply defined edges • Introduce superheated steam at a line pressure deposit metallic coatings on parts. This process is • Coating thickness can be closely controlled by of 35 to 105 kPa (5 to 15 psi) and allow furnace also known as mechanical plating, or peen plating, varying the number of revolutions the part is to purge for at least 15 min when the coating is less than 25 lxm (1 mil) thick. permitted to make • Increase furnace temperature to desired set Coating is accomplished by placing the workpiece, • An unlimited number of character forms can point and hold for no longer than 4 h at heat glass beads, water, and the metal plating powder in be applied at relatively low cost • Upon completion of cycle, reduce furnace tem- a tumbling barrel. perature to 315 °C (600 °F). When parts reach Zinc is most commonly used as a plating mate- Principal limitations of the process are: this temperature, the steam can be shut off and rial, although a wide range of metals and mixtures .. the parts unloaded of metals can be mechanically plated on ferrous • Special handling is required metal parts. For example, a mixture of 75% Zn • Internal surfaces are not coated and 25% Sn is commonly used. Metal powders Caution should be used when opening the fur- Types of paint used in general procedures for are added to the mixtures to be tumbled. Com- nace door after the steam cycle. As shown in painting of P/M parts are similar to those used for plete details on the mechanical coating process Equation 1, hydrogen is produced during this wrought counterparts, all of which are covered in may be found in the article "Mechanical Plating" process and can ignite. It is recommended that a detail in the article "Painting" in this Volume. in this Volume. nitrogen purge be applied prior to unloading. This Powder metallurgy parts can Powder metallurgy parts with densities not less Electroplating. process, when correctly applied, can impart im- be electroplated with various metals like their than 83% can be mechanically plated without proved surface properties, and, depending upon wrought counterparts. Methods used for plating of special considerations for porosity. When density steel composition, increased compressive yield cast parts generally can be used for very dense parts drops below 83%, tests should be conducted to strength. (95% or more of theoretical). Plating of castings is determine whether moisture is entrapped, which In all steam-treated P/M steels, the ductility is described in the article "Surface Engineering of Cast is detrimental to the finished parts. Generally, significantly reduced due to the internal stresses Irons" in this Volume. For parts of lower densities, when density is below 83%, parts must be im- created by the formation of the iron oxide. Care special preparation procedures are required. pregnated with wax or resin. must be taken when treating high-carbon P/M During plating of P/M parts, the pores act as steels because these internal stresses can initiate Painting. Usually, P/M parts are ideal candi- thermal pumps. Plating solutions are released microcracking and cause severe loss of ductility. dates for coating by painting; the porosity enhances from or absorbed by the pores, depending on the Many cases have been reported in which parts paint adhesion. Furthermore, P/M parts can be temperature differential between the workpiece were accidentally dropped on the floor after be- painted by spraying, dipping, or the contact transfer and the solution. Interconnecting pores entrap ing steam treated, and the parts subsequently method. Air-drying types are suitable only for in- solutions, which are then released slowly. Part shattered like glass. The best recommendation for door protective coatings. Baking produces finishes density should be known before the sequence of preventing such an incident is to specify a 0.5% of higher quality that are well suited for outdoor cleaning and plating operations begins, and suit- C (max) content on materials that are to be steam exposure. able precautions should be taken to prevent solu- treated. Spray painting has several advantages over tion entrapment. Figure 2(a) shows that transverse rupture dipping, including improved control of dimen- Entrapped solutions not only cause spotty plat- strength increases proportionately with sin- sions and coating quality, and the ability to coat ing and staining, which may develop within days, tered density. Upon steam treating, low-carbon localized areas. However, higher labor costs and but also can cause contamination and depletion of P/M steels exhibit a uniform increase in more paint loss from overspray are associated all solutions used in the production process. It is strength, whereas the high-carbon P/M steels with spraying. therefore necessary that, if part density is below show only a small incremental increase in For spraying, baking types of alkyds are re- about 95%, pores must be closed before coating strength. duced with solvents to a spraying viscosity of 35 by electroplating. Apparent hardness also is improved as shown s through a No. 4 Ford Cup. Parts are sprayed and Methods of closing pores that have proved suc- in Fig. 2(b). By filling the porosity with a hard then air dried for 10 min, after which they may be cessful include burnishing, buffing, rolling, heat second phase, the P/M steel can offer better sup- baked for 30 min at a temperature compatible treating, steam treating, and impregnation. All of port to an indentation hardness test. As with rup- with the type of paint being used. This practice these methods, except impregnation, provide ture strength, the incremental increase in hard- results in a dry film coating 38 to 46 lttm (1.5 to varying degrees of closure. Mechanical methods ness of high-carbon steels is less than that of 1.8 mils) thick. are often excluded because of dimensional toler- low-carbon steels. Additional information on the In paint dipping, the parts to be coated are ances. Consequently, impregnation is the most properties of steam-treated P/M steels can be placed in baskets or on racks, immersed in the suitable approach to closing pores. found in Ref 6. paint, and then allowed to drain. Dipping saves Infiltration of iron compacts with metals such labor and paint, compared to spraying, but gen- as copper is common practice and completely eral quality of dipped parts is lower, notably be- solves the porosity problem for subsequent elec- Coating of P/M Parts (Ref 4) cause of edge buildup. troplating. However, the cost of metal infiltration Roll painting and lithographing (transfer coat- usually cannot be justified only to ensure satis- In addition to the surfaces provided by steam ing) is a process in which paint is applied to factory electroplating. treatment, P/M parts are frequently coated by external surfaces of cylindrical P/M parts, fol- Impregnation with plastic seals P/M parts for mechanical means, painting, or electroplating. lowed by the application of lithographing ink. further processing, such as electroplating. Pres- The blue-black oxide-covered surface pro- Typically parts are roll coated and oven baked. sure tightness and frequently an improvement in duced by exposure to steam is often the final The initial coating is usually a background color. machining characteristics are added benefits de- finish for a variety of hardware items. It may Numbers of characters, as required, are then roll rived from plastic impregnation. The process is also be used as a preliminary coating for a final coated over the background coating, followed by not unlike the plastic impregnation process used finishing process, such as painting. baking dry. to attain pressure tightness in porous castings. 766 / Surface Engineering of Irons and Steels

Optimum results are obtainable with various Carbonitriding is probably the more common heat is rapidly dissipated, a rapid transfer to the types of plastic sealants, although the most com- case-hardening treatment used on P/M parts. Here quench is mandatory. monly used are polyester resins and anaerobic process temperatures are lower (800 to 850 °C, or As with wrought steels, the response to harden- sealants. 1470 to 1560 °F) and ammonia additions to ap- ing by induction is dependent upon combined A typical processing cycle consists of: proximately 10% are made. Ammonia dissociates carbon content, alloy content, and surface decar- on the parts, allowing nitrogen to diffuse into the burization. This latter variable can be a major • Cleaning thoroughly surfaces. This retards the critical cooling rate upon concern with P/M parts. With today's conven- • Baking at 120 to 150 °C (250 to 300 °F) to quenching and provides a more consistent marten- tional belt-type sintering furnaces using an en- drive off all moisture or solvent site transformation. It also produces a more consis- dogas atmosphere, decarburizing can occur as the • Applying sealant under vacuum, such as in an tent surface hardness, which improves wear parts leave the hot zone and cool slowly through autoclave resistance and toughness of the P/M steel. Because the 1100 to 800 °C (2010 to 1470 °F) temperature • Removing excess sealant by means of an emul- lower temperatures can be used, carbonitriding pro- range. sion cleaner vides better control of distortion compared to car- In most cases, P/M parts are quenched in a • Curing at 120 to 150 °C (200 to 250 °F) burizing. Care must be taken when adding water-based solution containing some type of rust • Tumbling, polishing, or abrasive blasting to ammonia, however, since excessive nitrogen diffu- preventative to forestall internal corrosion. In remove excess cured sealant sion into the intemal pore surfaces can cause embrit- those applications where induction hardening is flement. considered, densities above 90% should be speci- Parts are now ready for routine cleaning and Carbonitriding is a shallow case-hardening fied. With a decrease in density, the resistivity of plating cycles, as required by the plating method treatment. Case depths greater than 0.50 mm the steel increases and permeability decreases. used. Plating procedures are the same as those (0.020 in.) deep are seldom specified. For this For this reason, integral quench coils using a used for wrought parts (see the articles contained reason, cycle times are relatively short, usually on high-velocity spray quench are generally used to in the Section "Plating and Electroplating" in this the order of 30 to 60 min. As in neutral hardening, attain maximum surface hardness in the P/M part. Volume). carbon control is a critical aspect of the treatment. Nitrocarburizing. This process is rapidly Normally carbon potentials of 1.0 to 1.2% are growing in popularity as a treatment for P/M parts. Case Hardening of P/M Parts (Ref 5) specified to maintain the carbon profile in the Here, nitrogen is diffused into the surfaces of the Powder metallurgy parts can be case hardened part. steel in sufficiendy high concentration to form a thin by several processes, although various available lempering is usually required after case hard- layer of e iron nitride on the surface of the part. This processes are not equally suited to every applica- ening when densities exceed 90%. In this case, is done at temperatures ranging from 570 to 600 °C tion. Generally the best results (a clear case/core significantly high stresses that could initiate crack- (1060 to 1110 °F). At these temperatures no relationship) are obtained with P/M parts with ing are developed upon quenching. As porosity austenite transformation occurs, thereby signifi- densities exceeding 7.2 g/cm 3 (90% of theoretical increases, this stress level is reduced to a level at cantly reducing the dimensional changes and distor- density). More detailed information on each of which a posttemper is not necessary. However, tion. the processes described below can be found in judgment should be used when deciding whether The process uses conventional integral quench Heat Treating, Volume 4 of the ASM Handbook. tempering is required. If a substantial amount of atmosphere furnaces. The atmosphere usually Carburizing is normally specified in parts with retained austenite is formed upon carbonitriding, a consists of a 50/50 mixture of endothermic gas a large cross-sectional thickness to attain maximum temper is advisable. and anhydrous ammonia. Control of the nitrided fatigue and impact properties. The material usually If the part has thin cross sections, sharp cor- layer thickness, as with the other treatments, is specified for carburizing contains hardenability ners, or undercuts that would act as stress raiser, dependent on density. If the nitrided layer is al- agents such as nickel, molybdenum, and copper then tempering would also be advisable. Recom- lowed to form on the internal pore surfaces to any with relatively low carbon content. To develop op- mended tempering temperatures for P/M parts significant extent, a volume expansion can occur. timum dynamic properties at porosity levels be- range from 105 to 200 °C (220 to 390 °F). Above For this reason, density of the P/M part should be tween 10 to 15%, a combined carbon level of 0.30 this temperature, entrained quench oil can ignite, above 90% of theoretical. This nitrided layer, to 0.35% is recommended. As porosity is reduced creating a hazardous condition in the fumace. when properly applied, can reduce the coefficient below 10%, combined carbon can be reduced to Tempering above 200 °C (390 °F) will result in of friction at the surface of the part and provide 0.15 to 0.25% C. Because improved dynamic prop- improved toughness and fatigue properties of the improved wear resistance compared to conven- erties are also associated with high densities, it is heat-treated P/M steel. However, furnaces will tional hardening to martensite. This process is recommended that combined carbon be adjusted to need special adaptations to handle the high vol- best applied to applications where sliding wear a level best suited for re-pressing after sintering. ume of smoke created by the ignition of the and fretting are involved. In wrought steel, carburizing is normally char- quench oil. Because the hard nitrided layer is relatively acterized by a surface hardness range and an Spur gears, bevel thin, this process should not be applied where effective case depth. Microhardness measure- Induction Hardening. gears, splined hubs, and cams are ideal components high indentation or impact loading is involved. ments can accurately show the hardness profile in The ~ nitride layer that is formed can attain a file wrought steel but can be erratic when used on to utilize P/M production techniques. These parts P/M steels. With P/M steels, however, subsurface usually require hard wear-resistant surfaces in some hardness in excess of HRC 60, depending on the porosity can influence the microhardness read- areas, with the retention of the ductility of the sin- alloy content of the steel. Indentation hardness ings, resulting in false hardness readings. It is tered matrix in the remainder of the part. Induction testing is not recommended when evaluating this recommended that at least three hardness read- hardening is commonly specified for these applica- process. Since no transformation occurs, the P/M ings be taken at each level below the surface and tions. parts can be air cooled without loss of surface averaged to determine effective case depth. This process can be placed in an automated hardness. Also, no oil absorption occurs, which Carburizing of P/M steels is usually done at machining line that can reduce handling and be a leaves the porosity open for impregnation if de- temperatures between 900 to 930 °C (1650 to cost-effective hardening treatment when high sired. 1705 °F). Time cycles are normally short because volumes of parts are being produced. Because the Nitrocarburizing also provides improved of the rapid diffusion of carbon through the inter- inductance of P/M materials is typically reduced strength and reduced notch sensitivity in P/M connected porosity. Therefore, atmosphere car- due to porosity, a higher power setting is nor- parts. Figure 3 shows the fatigue improvement of bon potentials need to be somewhat higher than mally required to reach a given depth of harden- two low-carbon P/M steels after nitrocarburizing. those required for wrought steels of similar com- ing compared to that used for a wrought material A typical nitrocarburized microstructure of an position. of similar composition. Furthermore, because the iron-copper-carbon P/M steel is shown in Fig. 4. Surface Engineering of Specialty Steels / 767

300 44

250 36

~ 200 Nitrocarburized---29 .ff

(D 150 22 == ._~ 100 ~5 ~. As-sintered 50 7.3

0 0 105 106 107 108 Cycles (a)

300 44 Fig. 4 Typical microstructure in a sintered ferritic nitro- - Nitrocarburized carburized iron-copper-carbon P/M steel, l OOx. 250 36 Source: Ref 5 Fig, 5 Microstructure of a plasma nitrocarburized P/M steel with a compound surface layer thickness of 200 29 10 l.tm. Source: Ref 7 .ff .ff material, but close to the surface, also show the

e- presence of the compound layer. The extent of the C 150 22 ~ depth of such nitrocarburized pores is a function of the degree of interconnected porosity of the without breaking and without undergoing exces- As-sintered ._~ component, which is, in turn, a function of the sive wear or deformation. In many applications, ~ 100 15 y_ pressing conditions. tool steels must provide this capability under con- Ion Nitriding. The hardness, wear resistance, ditions that develop high temperatures in the tool. and fatigue strength can also be improved by Most tool steels are wrought products, but preci- 7.3 50 plasma, or ion, nitriding. This is a method of surface sion castings can be used in some applications. hardening using glow discharge technology to in- The powder metallurgy process is also used in 0 0 troduce nascent (elemental) nitrogen to the surface making tool steels. It provides, first, a more uni- 105 106 107 108 of a metal part for subsequent diffusion into the form carbide size and distribution in large sec- Cycles material. In a vacuum, high-voltage electrical en- tions and, second, special compositions that are (b) ergy is used to form a plasma, through which nitro- difficult or impossible to produce in wrought or Fig. 3 Increase in the notched axial fatigue strength of gen ions are accelerated to impinge on the cast alloys. sintered low-carbon P/M steels after nitrocarburiz- workpiece. This ion bombardment heats the working for 2 h at 570 °C (1060 °F). (a) F-O000 carbon steel. (b) FC-0205 copper-carbon steel. Metal powder density was piece, cleans the surface, and provides active nitro- Tool Steel Classifications 7.1 g/cm 3 (0.256 Ib/in.3). Source: Ref 5 gen. Ion nitriding provides better control of case Tool steels are classified according to their chemistry and uniformity and has other advantages, such as lower part distortion than conventional gas composition, applications, or method of quench- Plasma nitrocarburizing is in essence a variant nitriding. ing. Each group is identified by a capital letter; of the now well-established glow-discharge When ion nitriding of P/M steels, precleaning individual tool steel types are assigned code num- plasma (ion) nitriding method (see the discussion is more critical than with wrought alloys because bers. Table 2 gives composition limits for the tool that follows on ion nitriding). A technical argu- of the porosity characteristic. A baking operation steels most commonly used. More detailed infor- ment against the use of plasma nitrocarburizing should precede the ion nitriding of P/M parts in mation on tool steels, including their processing, has been the effect of retained lubricant on the order to break down or release agents and/or to properties, and applications, can be found in Ref character and stability of the glow-discharge evaporate any cleaning solvents. 8 and 9. plasma, thus effecting the reliability of the plasma High-speed steels are tool materials devel- technology when applied to sintered parts. Lubri- oped largely for use in high-speed metal cutting cants are added to powdered products in order to applications. There are two classifications of high- Tool Steels achieve optimum pressing conditions. A method speed steels; molybdenum high-speed steels, or by which the lubricant can be satisfactorily re- A tool steel is any steel used to make tools for group M, which contain from 0.75 to 1.52% C and moved prior to the P/M parts entering the vacuum cutting, forming, or otherwise shaping a material 4.50 to 11.0% Mo, and tungsten high-speed steels, chamber of the plasma unit is described in Ref 7. into a final part or component. These complex or group T, which have similar carbon contents but Using this method, it is now routinely possible to alloy steels, which contain relatively large high (11.75 to 21.00%) tungsten contents. Group M plasma nitrocarburize in one batch up to 4500 amounts of tungsten, molybdenum, vanadium, steels constitute more than 95% of all high-speed components, such as chain gear wheels, that have manganese, and/or chromium, make it possible to steel produced in the United States. been manufactured by P/M. The microstructure meet increasingly severe service demands. In Hot-work steels (group H) have been devel- of such a plasma nitrocarburized component is service, most tools are subjected to extremely oped to withstand the combinations of heat, pres- shown in Fig. 5. It is interesting to note that high loads that are applied rapidly. The tools must sure, and abrasion associated with punching, detailed examination shows that pores within the withstand these loads a great number of times shearing, or forming of metals at high temperatures. 768 / Surface Engineering of Irons and Steels

Table 2 Composition limits of principal types of tool steels

Designations Composition(a), % AISI SAE UNS C Mn Cr Ni Mo Co

Molybdenum high-speed steels M1 M1 T11301 0.78-0.88 0.15-0.40 0.20-0.50 3.50-4.00 0.30 max 8.20-9.20 1.40-2.10 1.00-1.35 ... M2 M2 T 11302 0.78-0.88; 0.95-1.05 0.15-0.40 0.20-0.45 3.75-4.50 0.30 max 4.50-5.50 5.50-6.75 1.75-2.20 ... M3, class 1 M3 T 11313 1.00-1.10 0.15-0.40 0.20-0.45 3.75-4.50 0.30 max 4.75-6.50 5.00-6.75 2.25-2.75 ... M3, class 2 M3 T11323 1.15-1.25 0.15-0.40 0.20-0.45 3.75-4.50 0.30 max 4.75-6.50 5.00-6.75 2.75-3.75 ... M4 M4 T 11304 1.25-1.40 0.15-0.40 0.20-0.45 3.75-4.75 0.30 max 4.25-5.50 5.25-6.50 3.75-4.50 M6 .. T11306 0.75-0.85 0.15-0.40 0.20-0.45 3.75-4.50 0.30 max 4.50-5.50 3.75-4.75 1.30-1.70 11.0()"i 3.00 M7 .. T11307 0.97-1.05 0.15-0.40 0.20-0.55 3.50-4.00 0.30 max 8.20-9.20 1.40-2.10 1.75-2.25 ... M10 .. T 11310 0.84-0.94; 0.95-1.05 0.10-0.40 0.20-0.45 3.75-4.50 0.30 max 7.75-8.50 1.80-2.20 M30 .. Tl1330 0.75-0.85 0.15-0.40 0.20-0.45 3.50-4.25 0.30 max 7.75-9.00 1.30"2.30 1.00-1.40 4.50-5.50 M33 .. Tl1333 0.85-0.92 0.15-0.40 0.15-0.50 3.50-4.00 0.30 max 9.00-10.00 1.30-2.10 1.00-1.35 7.75-8.75 M34 .. T11334 0.85-0.92 0.15-0.40 0.20-0.45 3.50-4.00 0.30 max 7.75-9.20 1.40-2.10 1.90-2.30 7.75-8.75 M36 .. T11336 0.80-0.90 0.15-0.40 0.20-0.45 3.75-4.50 0.30 max 4.50-5.50 5.50-6.50 1.75-2.25 7.75-8.75 M41 .. T 11341 1.05-1.15 0.20-0.60 0.15-0.50 3.75-4.50 0.30 max 3.25-4.25 6.25-7.00 1.75-2.25 4.75-5.75 M42 .. Tl1342 1.05-1.15 0.15-0.40 0.15-0.65 3.50-4.25 0.30 max 9.00-10.00 1.15-1.85 0.95-1.35 7.75-8.75 M43 .. T11343 1.15-1.25 0.20-0.40 0.15-0.65 3.50-4.25 0.30 max 7.50-8.50 2.25-3.00 1.50-1.75 7.75-8.75 M44 ... T11344 1.10-1.20 0.20-0.40 0.30-0.55 4.00-4.75 0.30 max 6.00-7.00 5.00-5.75 1.85-2.20 11.00-12.25 M46 ... T11346 1.22-1.30 0.20-0.40 0.40-0.65 3.70-4.20 0.30 max 8.00-8.50 1.90-2.20 3.00-3.30 7.80-8.80 M47 ... T11347 1.05-1.15 0.15-0.40 0.20-0.45 3.50-4.00 0.30 max 9.25-10.00 1.30-1.80 1.15-1.35 4.75-5.25 Tungsten high-speed steels T1 T1 T12001 0.65-0.80 0.10-0.40 0.20-0.40 3.75-4.00 0.30 max 17.25-18.75 0.90-1.30 T2 T2 T12002 0.80-0.90 0.20-0.40 0.20-0.40 3.75-4.50 0.30 max 1.00 max 17.50-19.00 1.80-2.40 T4 T4 T12004 0.70-0.80 0.10-0.40 0.20-0.40 3.75-4.50 0.30 max 0.40-1.00 17.50-19.00 0.80-1.20 4.25~5.75 T5 T5 T12005 0.75-0.85 0.20-0.40 0.20-0.40 3.75-5.00 0.30 max 0.50-1.25 17.50-19.00 1.80-2.40 7.00-9.50 T6 T12006 0.75-0.85 0.20-0.40 0.20-0.40 4.00-4.75 0.30 max 0.40-1.00 18.50-21.00 1.50-2.10 11.00-13.00 • 8 ~8 T12008 0.75-0.85 0.20-0.40 0.20-0.40 3.75-4.50 0.30 max 0.40-1.00 13.25-14.75 1.80-2.40 4.25-5.75 T15 ... T 12015 1.50-1.60 0.15-0.40 0.15-0.40 3.75-5.00 0.30 max 1.00 max 11.75-13.00 4.50-5.25 4.75-5.25 Chromium hot-work steels H10 T20810 0.35-0.45 0.25-0.70 0.80-1.20 3.00-3.75 0.30 max 2.00-3.00 ... 0.25-0.75 Hll Hi'l T20811 0.33-0.43 0.20-0.50 0.80-1.20 4.75-5.50 0.30 max 1.10-1.60 0.30-0.60 H12 H12 T20812 0.30-0.40 0.20-0.50 0.80-1.20 4.75-5.50 0.30 max 1.25-1.75 1.00" i .70 0.50 max H13 H13 T20813 0.32-0.45 0.20-0.50 0.80-1.20 4.75-5.50 0.30 max 1.10-1.75 0.80-1.20 H14 ... T20814 0.35-0.45 0.20-0.50 0.80-1.20 4.75-5.50 0.30 max 4.00"5.25 HI9 ... T20819 0.32-0.45 0.20-0.50 0.20-0.50 4.00-4.75 0.30 max 0.30"0.55 3.75-4.50 1.75~2.20 4.00~.50 Tungsten hot-work steels I-I21 H21 T20821 0.26-0.36 0.15-0.40 0.15-0.50 3.00-3.75 0.30 max 8.50-10.00 0.30-0.60 H22 ... T20822 0.30-0.40 0.15-0.40 0.15-0.40 1.75-3.75 0.30 max 10.00-11.75 0.25-0.50 I-I23 ... T20823 0.25-0.35 0.15-0.40 0.15-0.60 11.00-12.75 0.30 max 11.00-12.75 0.75-1.25 H24 ... T20824 0.42-0.53 0.15-0.40 0.15-0.40 2.50-3.50 0.30 max 14.00-16.00 0.40-0.60 H25 ... T20825 0.22-0.32 0.15-0.40 0.15-0.40 3.75-4.50 0.30 max 14.00-16.00 0.40-0.60 H26 ... T20826 0.45-0.55(b) 0.15-0.40 0.15-0.40 3.75-4.50 0.30 max 17.25-19.00 0.75-1.25 Molybdenum hot-work steels H42 ... T20842 0.55-0.70(b) 0.15-0.40 ... 3.75-4.50 0.30 max 4.50-5.50 5.50-6.75 1.75-2.20 Air-hardening medium-alloy cold-work steels

A2 A2 T30102 0.95-1.05 1.00 max 0.50 max 4.75-5.50 0.30 max 0.90-1.40 ... 0.15-0.50

A3 ... T30103 1,20-1.30 0.40-0.60 0.50 max 4.75-5.50 0.30 max 0.90-1.40 ... 0.80-1.40

A4 T30104 0.95-1.05 1.80-2.20 0.50 max 0.90-2.20 0.30 max 0.90-1.40 ...... A6 T30106 0.65-0.75 1.80-2.50 0.50 max 0.90-1.20 0.30 max 0.90-1.40 A7 T30107 2.00-2.85 0.80 max 0.50 max 5.00-5.75 0.30 max 0.90-1.40 o5oi5o 396:; 15 A8 T30108 0.50-0.60 0.50 max 0.75-1.10 4.75-5.50 0.30 max 1.15-1.65 1.00-1.50

A9 T30109 0.45-0.55 0.50max 0.95-1.15 4.75-5.50 1.25-1.75 1.30-1.80 ... o8oi4o

A10 T30110 1.25-1.50(c) 1.60-2.10 1.00-1.50 ... 1.55-2.05 1.25-1.75 ...... High-carbon, high-chromium cold-work steels D2 D2 T30402 1.40-1.60 0.60 max 0.60 max 11.00-13.00 0.30 max 0.70-1.20 ... 1.10 max 1.00 max

D3 D3 T30403 2.00-2.35 0.60 max 0.60 max 11.00-13.50 0.30 max 1.00 max 1.00 max ... 134 T30404 2.05-2.40 0.60 max 0.60 max 11.00-13.00 0.30 max 0.70-i.20 ... 1.00 max D5 D5 T30405 1.40-1.60 0.60 max 0.60 max 11.00-13.00 0.30 max 0.70-1.20 ... 1.00 max 25~35o

D7 D7 T30407 2.15-2.50 0.60 max 0.60 max 11.50-13.50 0.30 max 0.70-1.20 ... 3.80-4.40 ... Oil-hardening cold-work steels O1 O1 T31501 0.85-1.00 1.00-1.40 0.50 max 0.40-0.60 0.30 max ... 0.40-0.60 0.30 max 02 02 T31502 0.85,0.95 1.40-1.80 0.50 max 0.35 max 0.30 max 0.30 max ... 0.30 max

06 06 T31506 1.25-1.55(c) 0.30-1.10 0.55-1.50 0.30 max 0.30 max 0.20-0.30 ... 07 ... T31507 1.10-1.30 1.00 max 0.60 max 0.35-0.85 0.30 max 0.30 max 1.00"2.00 0.40 max

(a) All steels except group W contain 0.25 max Cu, 0.03 max P, and 0.03 max S; group W steels contain 0.20 max P, and 0.025 max S. Where specified, sulfur may be increased to 0.06 to 0.15% to improve machinability of group H, M, and T steels. (b) Available in several carbon ranges. (c) Contains free graphite in the microstructure. (d) Optional. (e) Specified carbon ranges are designated by suffix numbers. (continued) Surface Engineering of Specialty Steels / 769

Table 2 Composition limits of principal types of tool steels (continued)

Designations Composition(a), % AISI SAE UNS C Mn Si Cr Ni Mo W V Co

Shock-resisting steels S1 S1 T41901 0.40-0.55 0.10-0.40 0.15-1.20 1.00-1.80 0.30 max 0.50 max 1.50-3.00 0.15-0.30 $2 $2 T41902 0.40-0.55 0.30-0.50 0.90-1.20 ... 0.30 max 0.30-0.60 ... 0.50 max $5 $5 T41905 0.50-0.65 0.60-1.00 1.75-2.25 0.35 max ... 0.20-1.35 ... 0.35 max $6 ... T41906 0.40-0.50 1.20-1.50 2.00-2.50 1.20-1.50 ... 0.30-0.50 ... 0.20-0.40 $7 ... T41907 0.45-0.55 0.20-0.80 0.20-1.00 3.00-3.50 ... 1.30-1.80 ... 0.20-0.30(d) Low-alloy special-purpose tools steels L2 T61202 0.45-1.00(b) 0.10-0.90 0.50 max 0.70-1.20 0.25 max O. 10-0.30 L6 /)6 T61206 0.65-0.75 0.25-0.80 0.50 max 0.60-1.20 1.25~2.00 0.50 max o.2o-0.3o(a) Low-carbon mold steels

... T51602 0.10 max 0.10-0.40 0.10-0.40 0.75-1.25 0.10-0.50 0.15-0.40

.. T51603 0.10 max 0.20-0.60 0.40 max 0.40-0.75 1.00-1.50

P4 .. T51604 0.12 max 0.20-0.60 0.10-0.40 4.00-5.25 0.40~].00

1~ .. T51605 0.10 max 0.20-0.60 0.40 max 2.00-2.50 0.35"max ...

P6 .. T51606 0.05-0.15 0.35-0.70 0.10-0.40 1.25-1.75 3.25-3.75 P20 .. T51620 0.28-0.40 0.60-1.00 0.20-0.80 1.40-2.00 0.30"0.55 P21 .. T51621 0.18-0.22 0.20-0.40 0.20-0.40 0.20-0.30 3.90~t.25 ... 0.15~).25 1.05-i:25A1 Water-hardening tool steels W1 W108, W109, T72301 0.70-1.50(e) 0.10-0.40 0.10-0.40 0.15 max 0.20 max 0.10 max 0.15 max 0.10 max Wll0,Wll2 W2 W209, W210 T72302 0.85-1.50(e) 0.10-0.40 0.10-0.40 0.15 max 0.20 max 0.10 max 0.15 max 0.15-0.35 W5 ... T72305 1.05-1.15 0.10-0.40 0.10-0.40 0.40-0.60 0.20 max 0.10 max 0.15 max 0.10 max

Group H steels usually have medium carbon con- ing tools, woodworking tools, metal-cutting tools, to 1025 °F) for 2 to 4 h. A nitrided depth that ranges tents (0.35 to 0.45%) and combined chromium, and wear-resistant machine tool components. from 13 to 76 gtm (0.0005 to 0.003 in.) is desired. tungsten, molybdenum, and vanadium contents of 6 Because of decreased wear and die pickup, to 25%. H steels are divided into chromium hot- Surface Treatments for Tool Steels (Ref 10) cold-extrusion punches experience a two to work steels, tungsten hot-work steels, and molybde- three-fold improvement in life. Nitriding is often num hot-work steels. Most surface treatments are employed to in- used whenever mold wash is a problem in the die Cold-work steels are restricted in application crease surface hardness and/or wear resistance, casting of zinc or aluminum alloys. Galling of to those uses that do not involve prolonged or re- minimize adhesion (reduce friction), or improve sheet metal working dies can be alleviated by peated heating above 205 to 260 °C (400 to 500 °F). the corrosion resistance of the tool steel base. The nitriding these dies before use. There are three categories of cold-work steels: air- processes discussed below are described in Steels that will be nitrided should contain one hardening steels, or group A; high-carbon, high- greater detail elsewhere in this Volume or in Heat or more of the nitride-forming elements (chro- chromium steels, or group D; and off-hardening Treating, Volume 4 of the ASM Handbook. mium, vanadium, or aluminum) in order to pre- steels, or group O. Carburizing. The processes of case hardening vent the easy spalling and chipping that results Shock-resisting, or group S, steels contain and carburizing are of limited use in tool steel appli- when iron nitride is formed. Commonly nitrided manganese, silicon, chromium, tungsten, and mo- cations, because of the relatively high carbon con- tool steels include HI 1, H12, H13, A2, 02, and lybdenum, in various combinations; carbon content tents of the tool steels. Carburizing can be the high-speed tool steels. is about 1.50%. Group S steels are used primarily accomplished in many ways, and essentially con- Ion or plasma nitriding has many of the for chisels, rivet sets, punches, and other applica- sists of heating the final machined tool into the same characteristics of liquid or gas nitriding. This tions requiring high toughness and resistance to austenite region in the presence of carbonaceous process relies on a nitrogen gas being ionized by shock loading. solids, liquids, or gases. glow discharge conditions between the tool (car- The low-alloy special purpose, or group L, Low-carbon plastic mold steels (P type) are bide) and the furnace wall or shield (anode). The tool steels contain small amounts of chromium, often carburized after hubbing or machining of primary advantages are the reductions in time and vanadium, nickel, and molybdenum. Group L steels the cavity in the mold. In this application, the tool temperature, which save money and reduce the dis- are generally used for machine parts and other spe- steel is intentionally lean in carbon content to tortion and softening of prehardened tools. Usually, cial applications requiring good strength and tough- improve hubbing or machining, and must be car- treatment times vary between 0.5 and 36 h. ness. burized in order to have sufficient surface hard- Boriding. In this process, boron atoms from a Mold steels, or group P, contain chromium ness for the end use. solid, liquid, gas, or plasma atmosphere surrotmding and nickel as principal alloying elements. Because Nitriding is a frequently used surface treatment the finished part are dit~sed into the surface, creat- of their low resistance to softening at elevated tem- that increases surface hardness, adds to the corro- ing a hard, water-resistant iron boride layer. Metal- peratures, group P steels are used almost exclusively sion resistance of the tool, and reduces friction. to-metal wear testing demonstrated a three-fold in low-temperature die casting dies and in molds for Basically, the process involves heating the finished improvement in wear resistance of borided O 1 and injection or compression molding of plastics. tool in the presence of a nitrogen-containing liquid 02 tool steels and over a two-fold increase in A2 Water-hardening, or group W, tool steels or gas and allowing nitrogen to diffuse into the tool. tool steel (Ref 11). Borided A2 tool steel showed contain carbon as the principal alloying element Gas nitriding is usually accomplished at a lower twice the life of uncoated 02 tool steel in a deep- (0.70 to 1.50% C). Group W steels, which also have temperature (about 527 °C, or 980 °F) and longer drawing operation in which low-carbon steel cups low resistance to softening at elevated temperatures, time (10 to 90 h) than liquid nitriding, which occurs were manufactured (Ref 11). An H13 roller de- are suitable for cold heading, coining, and emboss- at temperatures ranging from 538 to 552 °C (1000 signed to flange milk cans was borided and pro- 770 / Surface Engineering of Irons and Steels

Table 3 Machining tool life improvements due to steam oxidation Bright Finish. Most high-speed cutting tools are finished with a ground or mechanically polished Too! life surface that would be categorized as a bright finish. Tool Application Before steam treating After steam treating Bright finished tools are often preferred to tools with M2 broachers Cutting AIS11010 latch 20 h per grind 70 h per grind an oxide finish for machining nonferrous work ma- M2 drills Drilling Bakelite plastic insulating blocks 10 holes 25 holes terial. The smooth or bright finish tends to resist Phenolic terminal plates 1700 holes per grind 8500 holes per grind Drilling AIS14030 steel 25 mm (1 in.) thick 17 holes 81 holes galling, a type of welding or buildup associated with M7 end mill tools Cutting 8740 steel forgings 30 pieces 200 pieces many nonferrous alloys. However, work materials A6 hobs Cutting teeth on AISI 31 40 forged gear 62.2% increased life of ferrous alloys tend to adhere to similar, iron-base M2 milling cutters Two slots in 1020 steel 150 cuts per grind'-- 306 cuts per grind tools having a bright finish. This buildup on the Slotting 1020 steel bars 2000 per grind 7000 per grind M2 saw blades Cutting 75 mm (3 in.) rods, austenitic steel 100% endurance at 0.52 m/s 120% endurance at 0.57 m/s cutting edges leads to increased frictional heal poor (102 sfm) (112 sfm) surface finish, and increased load at the cutting M2 taps Cutting SAE 52100 steel 1800 pieces 3000 pieces edge. Three different kinds of plating are Source: Ref 13 Plating. used on tool steels. Cadmium plating is used for appearance purposes and to reduce corrosion of the tool. It also has some usefulness in preventing adhe- Table 4 Effectof steam oxidation on tool life in forming various carbon steel nuts and bolts sion. Nickel plating is commonly used for appearance purposes and to prevent corrosion. Tool life The most commonly practiced tool steel plat- Tool Application Before steam treating(a) After steam treating(b) ing process is hard chromium plating. Plating M2 4th station punch Castle nut 1030 material 21,000 nuts 42,000 nuts thickness varies between 2 and 13 l.tm (0.0001 M2. 4th station punch Slotted insert nut 1030 material 22,000 nuts 38,000 nuts and 0.0005 in.) and, because it is very hard, it M2 4th station punch Castle nut 1030 material 29,000 nuts 80,000 nuts prolongs life by increasing abrasive wear resis- M2 3rd station punch Castle nut 1110 material 20,000 nuts 35,000 nuts M2 4th station punch Castle nut 1110 material 15,000 nuts 35,000 nuts tance. More important than plating hardness is its M2 trim die Bolt head 1335 material 7,000 bolts 16,000 bolts very low friction coefficient, which effectively prevents adhesive wear. (a) Hardened and triple tempered. (b) Hardened, triple tempered and steam treated. Source: Ref 13 However, hard chromium plating is not without problems. Tool steels may be hydrogen em- brittled when plated, and the plating has a ten- duced three times as many cans before it wore out and carbon in the substrate. The carbide layer dency to spall and flake. These wear debris can (Ref 12). thickens due to reaction between the carbide- actually accelerate abrasive wear. Boriding takes place at temperatures as low as forming element atoms in the salt bath and the Chemical vapor deposition (CVD), a proc- 600 °C (1100 °F), but usual practice involves a carbon atoms diffusing into the outside surface ess conducted in a vacuum chamber, relies on a period from 1 to 6 h at temperatures from 800 to layer from the interior of the substrate. deposition from reacted gas onto the tool steel sur- 900 °C (1470 to 1650 °F) (Ref 11, 12). The The thickness of the carbide layer is varied by face. Many different materials can be used as coat- resultant layer is between 13 and 130 I.tm (0.0005 controlling the bath temperature and immersion ings. Chromium, A1203, TiC, CrC, Fe4N, and TiN and 0.005 in.), and tends to be dull because of the time. An immersion time of 4 to 8 h is needed for are commonly used, and other materials are being microroughness of the surface. This high process H 13 steel to produce carbide layers with satisfac- studied. This process utilizes high temperatures, temperature requires that the boron treatment act tory thickness (5 to 10 l.tm) for die-casting appli- usually above 800 °C (1472 °F), which means that as the austenitization step, or else the process cations. Dies are then removed from the bath and tool steels must be tempered after the CVD coating must be followed by reaustenitization. This nec- cooled in oil and salt or air for core hardening is applied. The most popular wear-resistant coatings essarily limits the process to applications where followed by tempering. are TiC and TiN, which are used to coat high-speed, tolerances of about 25 l.tm (0.001 in.) can be Coated tool steels, such as H 12 and HI 3 steels, cold-work die and hot-work die tool steels. These tolerated. exhibit high hardness and excellent resistance to coatings commonly range in thickness from 2 to 20 Carbide Coating by Toyota Diffusion Proc- wear, seizure, corrosion, and oxidation. In addi- ~m (0.0001 to 0.001 in.). Using CVD coating with ess. Good surface covering and strongly bonding tion, resistance to cracking, flaking, and heat TiC and TiN, the primary mechanism of wear reduc- carbide coatings, such as VC, NbC, and Cr7C3, can checking is claimed. Hardness of the coating de- tion is the extremely high hardness, which leads to be formed on die steel surfaces by a coating method pends on layer composition: 3500 HV for vana- excellent abrasion resistance, although some de- developed at Toyota Central Research and Develop- dium carbide, 2800 HV for niobium carbide, and crease in friction coefficient can often be realized. ment Laboratory, Inc. of Japan. 1700 HV for chromium carbide. The chlorine content of the coating must carefully In the Toyota Diffusion (TD) process, metal Oxidation is a well-established process used dies to be treated are degreased, immersed in a for high-speed steel cutting tools. Increases in tool be maintained at a level below 5% to avoid degra- carbide salt bath for a specific time period, life of up to 100% are mostly due to a decrease in dation of the wear resistance (Ref 14). quenched for core hardening, tempered, and friction, because of the hard oxide coating and the Tool steels that can be successfully CVD washed in hot water for the removal of any resid- ability of the porous oxide to entrap lubricant and coated include the AISI A, S, D, H, M, and T steel ual salt. The borax salt bath contains compounds draw it to the tool-workpiece interface. Steam oxi- types. The lower-alloyed S type and all of the W (usually ferroalloys) with carbide-forming ele- dation of a finished tool is accomplished either by and O types are either very difficult or impossible ments such as vanadium, niobium, and chro- exposure to steam at a temperature of about 566 °C to properly coat, because of their low austenitiza- mium. The bath temperature is selected to con- (1050 °F) or by treating in liquid sodium hydroxide tion temperatures. form to the hardening temperature of the die steel. and sodium nitrate salts at approximately 140 °C Physical vapor deposition (PVD), which is For example, the borax bath temperature would (285 °F) for periods of time ranging from 5 to 20 also conducted in a vacuum chamber, can be accom- be between 1000 and 1050 °C (1830 and 1920 min. These treatments result in a black oxidized plished in several different ways. The process relies °F) for H 13 die steel. layer that is less than 5 ~trn (0.0002 in.) thick and will on plasma-aided precipitation of either TiC or TIN The carbide layer is formed on the die surface not peel, chip, nor crack, even when the tool is bent onto tool steel at temperatures ranging from 200 to through a chemical reaction between carbide- or cut. Tool life improvements due to steam oxida- 550 °C (400 to 1025 °F) (Ref 14). This temperature forming elements dissolved in the fused borax tion are listed in Tables 3 and 4. range is much more suitable for the coating of Surface Engineering of Specialty Steels / 771 high-speed tool steels than the temperatures re- Table 5 Increasedtool life attained with PVD coated cutting tools quired for CVD. Tool steel wear is reduced in about the same Cutting tool Workpieces machined High-speed before resharpening proportions (2 to 6 times less wear), whether the Type tool steel, AISI type Coating Workpiece material Uncoated Coated TiC or TiN is applied by CVD or PVD. TiN coatings on H 13 pins reduced the friction coeffi- End mill M7 TiN 1022 steel, 35 HRC 325 1200 End mill M7 TiN 6061-T6 aluminum alloy 166 1500 cient in pin-on-disk tests from 0.7 to less than 0.2 End mill M3 TiN 7075T aluminum alloy 9 53 (Ref 15). Modified ASTM G65-10 abrasive wear Gear hob M2 TiN 8620 steel 40 80 testing of D3 steel showed that wear of the TiN- Broach insert M3 TiN Type 303 stainless steel 100,000 300,000 coated samples was between 4 and 23% of the Broach M2 TiN 48% nickel alloy 200 3400 Broach M2 TiN Type 410 stainless steel 10,000-12,000 31,000 uncoated samples, depending on their initial sur- Pipe tap M2 TiN Gray iron 3000 9000 face roughness (Ref 15). This result led Tap M2 TiN 1050 steel, 30-33 HRC 60-70 750-800 Sundquist et al. to propose that increases in tool Form tool T 15 TiC 1045 steel 5000 23,000 life that are due to TiN coating can only be ex- Form tool T 15 TiN Type 303 stainless steel 1840 5890 pected when the surface roughness is less than the Cutoff tool M2 TiC-TiN Low-carbon steel 150 1000 Drill M7 TiN Low-carbon steel 1000 4000 coating thickness (Ref 15). Specific examples of Drill M7 TiN Titanium alloy 662 layered with 9 86 the use of PVD coatings for improving the life of D6AC tool steel, 48-50 HRC high-speed steel tools are listed in Table 5. Ion implantation is a process by which atoms Source: Ref 16 of virtually any element can be injected into the near-surface region of any solid. The implantation process involves forming a beam of charged ions of the desired element and then accelerating them at Table 6 Examplesof ion implantation in metalforming and cutting applications high energies towards the surface of the solid, which Energy, is held under high vacuum. The atoms penetrate into Part Part material Process Work material Ion keY Benefit the solid to a depth of 0.25 to 25 nm (2.5 to 250/~). Tool inserts TiN-coated tool Machining 4140 N 80 3x life This process differs from coating processes in that it steel does not produce a discrete coating; rather, it alters Taps HSS Tapping 41 40 N 80 3x life the chemical composition near the surface of the HSS Tapping 4130 N 80 5x life solid. The most common element implanted in tool HSS Tapping 41 40 N 50 10x life M35 Tapping ... N2 200 4x life steels in order to improve tribological properties, M7 Tapping N 100 2x life specifically adhesive and sliding wear, is nitrogen. Cutting blade M2 Cutting 1050 N 100 2x life Examples of ion implantation in metal forming and M2 Cutting SAE 950 N 100 4x life cutting applications are listed in Table 6. Ion implan- Dies D2 Forming 321 SS N 80 2x life M2 Forming Steel N 100 2-12x life tation of titanium and carbon has also improved the M2 Forming 1020 N 100 Negligible effect service life of stamping and cutting tools. D6 Forming TiO2 and rubber N 100 6x life laser surface processing methods, such as Molds D2 Forming Polymers N 50 5x life laser melting, have also been applied to tool steels. Rollers H 13 Rolling Steel N 100 5x life Hsu and Molian (Ref 18) reported that the tool life Note: HSS, high-speed steel; SS, stainless steel. Source: Ref 17 of laser-melted M2 steel tool bits was from 200 to 500% higher than if they were conventionally hardened using catastrophic failure criterion (Fig. 6). For laser-melted M35 steel tool bits, the tool life was 1030 to 2420 MPa (150 to 350 ksi). Some experi- A number of cobalt-free maraging steels and a from 20 to 125% higher than if they were conven- mental maraging steels have yield strengths as low-cobalt bearing maraging steel have recently tionally hardened using flank wear failure criterion high as 3450 MPa (500 ksi). These steels typi- been developed. The driving force for the devel- (Fig. 6). High-alloy martensite, fine austenite grain cally have very high nickel, cobalt, and molybde- opment of these particular alloys was the cobalt size, and finely dispersed carbides all contributed to num contents and very low carbon contents. shortage and resultant price escalation of cobalt high hardness, good toughness, and low coefficient Carbon, in fact, is an impurity in these steels and during the late 1970s and early 1980s. The nomi- of friction. is kept as low as commercially feasible in order nal compositions for these alloys are also listed in to minimize the formation of titanium carbide Table 7. (TIC), which can adversely affect strength, duc- Maraging Steels (Ref 19) tility, and toughness. Table 7 lists the chemical compositions of the Surface Treatments for Maraging Maraging steels comprise a special class of more common grades of maraging steel. The no- Steels (Ref 19) high-strength steels that differ from conventional menclature that has become established for these steels in that they are hardened by a metallurgical steels is nominal yield strength (ksi units) in pa- reaction that does not involve carbon. Instead, rentheses. Thus, for example, 18Ni (200) steel is Cleaning. Grit blasting is the most efficient these steels are strengthened by the precipitation normally age hardened to a yield strength of 1380 technique for removing oxide films formed by heat of intermetallic compounds at temperatures of MPa (200 ksi). The first three steels in Table treatment. Maraging steels can be chemically about 480 °C (900 °F). The term maraging is 7--18Ni (200), 18Ni (250), and 18Ni (300)----are cleaned by pickling in sulfuric acid or by duplex derived from martensite age hardening and de- the most widely used and most commonly avail- pickling in hydrochloric acid and then in nitric acid notes the age hardening of low-carbon, iron- able grades. The 18Ni (350) grade is an ultrahigh- plus hydrofluoric acid (see Tables 7 and 11 in the nickel lath martensite matrix. The physical met- strength variety made in limited quantities for article "Surface Engineering of Stainless Steels" in allurgy and properties of maraging steels are special applications. Two 18Ni (350) composi- this Volume). As with conventional steels, care must described in Ref 19. tions have been produced (see the footnote in be taken to avoid overpickling. The sodium hydride Commercial maraging steels are designed to Table 7). The 18Ni (Cast) grade was developed cleaning of maraging steels should be avoided to provide specific levels of yield strength from specifically as a cast composition. minimize problems with crack formation. Grease 772 / Surface Engineering of Irons and Steels

r 1 0 2 | | | l | | | | | 2.5 I I I E 0 Untreated o O Untreated tM -- 0 Melted i -- O- Melted o r- x 0

E k. / 0 10 1.5 0" 10" .0 ~ f / "0 t-- o t~ o / I-- te- e- M2 steel eD M35 t0ol steel c~ Feed rate: 0.01 cm/rev Feed rate: 0.01 cm/rev t., (i) Depth of cut: 0.1 cm Depth of cut: 0.1 cm > < I .... i .... t ' ' ' 0.5 I I I 35 40 45 50 0 50 100 150 200 Cutting speed (m/min) Test time (min) (a) (b)

Fig. 6 Tool life of conventionally heat-treated and laser-melted tool bits. (a) M2 tool steel. (b) M35 tool steel. Source: Ref 18

Table 7 Nominal compositions of commercial maraging steels generators, and transformers. The beneficial ef- fects of silicon additions to iron include: Composition, %(a) Grade Ni Mo Co Ti AI Nb • Increase of electrical resistivity • Suppression of the 7 loop enabling desirable Standard grades grain growth 18Ni(200) 18 3.3 8.5 0.2 0.1 18Ni(250) 18 5.0 8.5 0.4 0.1 • Development of preferred orientation grain 18Ni(300) 18 5.0 9.0 0.7 0.1 structure 18Ni(350) 18 4.2(b) 12.5 1.6 0.1 18Ni(Cast) 17 4.6 10.0 0.3 0.1 The addition of silicon also reduces magne- 12-5- 3( 180)(c ) 12 3 ... 0.2 0.3 tocrystalline anisotropy energy, and at-6.5% Si Cobalt-free and low-cobalt bearing grades content reduces the magnetostriction constants to Cobalt-free 18Ni(200) 18.5 3.0 ... 0.7 0.1 nearly zero. High-permeability and low hystere- Cobalt-free 18Ni(250) 18.5 3.0 .._ 1.4 0.1 sis losses can therefore be attained at the 6.5Si-Fe Low-cobalt 18Ni(250) 18.5 2.6 2.0 1.2 0.1 0~i composition. On the negative side, the addition of Cobalt-free 18Ni(300) 18.5 4.0 ... 1.85 0.1 .°. silicon to iron lowers magnetic saturation, lowers (a) All grades contain no more than 0.03% C. (b) Some producers use a combination of 4.8% Me and 1.4% Ti, nominal. (c) Contains 5% Curie temperature, and seriously decreases me- Cr chanical ductility. At silicon levels above ~4%, the alloy becomes brittle and difficult to process by cold-rolling methods; thus, few commercial and oils can be removed by cleaning in trichlo- Salt bath nitriding for 90 min at 540 °C (1000 °F) steels contain more than -3.5% Si. roethane-type solutions. has been done successfully. Such treatment must be The commercial grades of silicon steel in com- Nickel Plating. Mamging steels can be nickel very carefully controlled to avoid excessive overag- mon use are made mostly in electric or basic plated in chloride baths provided that proper sur- ing. Both the fatigue strength and the wear resis- oxygen furnaces. Continuous casting and/or vac- face-activation steps are followed. Heavy chro- tance (Fig. 7) of maraging steels are improved by uum degassing (V-D) may be employed. Flat- mium deposits can be plated on top of nickel nitriding. rolled silicon-iron sheet and strip has low sulfur electrodeposits. Maraging steels are less susceptible Maraging steels can also be surface hardened content, typically below 0.25%, with better to hydrogen embritflement during plating than con- by ion nitriding. Ozbaysal and Inal (Ref 20) have grades below 0.01%. Carbon contents are fre- ventional quenched and tempered steels of compa- demonstrated that the surface hardening of ma- quently less than 0.04%. Manganese may be rable hardness. They are not immune to hydrogen, raging steels without a reduction in core hardness present up to approximately 0.70%. Residual ele- however, and baking after plating is recommended. is possible using the ion nitriding process. Their ments such as chromium, molybdenum, nickel, Baking should be done at temperatures of about 150 studies on 18Ni(250), 18Ni(300), and 18Ni(350) copper, and phosphorus may also be present. The to 205 °C (300 to 400 °F) for periods of 3 to 10 h, showed that the highest surface hardness and the major alloying addition is silicon plus up to 0.6% depending on size and baking temperature. Baking highest core hardness for all three grades were A1 (optional). These alloys are not generally sold cannot be combined with age hardening, because achieved by nitriding at approximately 440 °C on the basis of their composition, but rather are considerable hydrogen remains in the steel after heat (825 °F). Figure 8 shows the surface and core sold based upon controlled magnetic properties, treating at the higher temperatures. hardness as functions of ion nitriding time and particularly ac core losses. Nitriding. Considerable surface hardening can temperature for 300-grade maraging alloy. Electrical sheet grades are divided into two be achieved by nitriding maraging steels in dissoci- general classifications, (1) oriented steels and (2) ated ammonia. Hardness levels equivalent to 65 to nonoriented steels. The oriented steels are given 70 HRC can be achieved at depths of up to 0.15 mm Electrical Steels (Ref 21) mill treatments designed to yield exceptionally (0.006 in.) after nitriding for 24 to 48 h at 455 °C good magnetic properties in the rolling, or length- (850 °F). Nitriding at this temperature allows age Electrical steels are fiat-rolled silicon-contain- wise, direction of the steel. Nonoriented grades hardening to occur during nitriding; therefore, the ing alloys used for soft magnetic applications are made with a mill treatment that yields a grain two processes can be accomplished simultaneously. such as components (magnetic cores) for motors, structure, or texture, of a random nature and, Surface Engineering of Specialty Steels / 773

0.50 0.020 core loss is obtained with higher silicon contents, 100. If the core-loss value is expressed in Mara:~/HI3///" ., larger grain size optimization, lower impurity watts/kg, the grade designation takes the suffix 0.40 0.016 E levels, thinner gages, and insulating coatings. M, indicating an ASTM metric standard. Several E ._ The AISI designations are still in common use, ASTM fiat-rolled products specifications are x" 0.30 0.012 ~£ but the newer ASTM designations provide more written in English and metric versions, such as A "10 "13 t-- .~_ specific information regarding the grade identi- 677-84 and its companion metric specification A "= 0.20 0.008 fied. A typical ASTM designation is 47S200. The 677M-83. nitrided first two digits of the ASTM designation indicate • 0.10 0.004 ~ ~.I" Maraging the thickness in mm (x 100). Following these dig- nitrided Surface Treatments for Electrical Steels I 0 its is a letter (C, D, F, S, G, H, or P) that indicates 0 200 400 600 800 1000 the material type and the respective magnetic test The purpose of the core metal in a motor, gen- Number of forgings conditions. The last three digits provide an indi- erator, or transformer is to offer the best path for cation of the maximum allowable core loss in the magnetic lines of flux, and its success in this Fig. 7 Relative wear rates of nitrided and non-nitrided units of either (watts&g) x 100, or (watts/lb) x respect is measured by its permeability. Cores are tool steels and maraging steels used in extrusion forging usually composed of a larger number of thin

therefore, the magnetic properties in the rolling Table 8 Properties of selected electrical steels direction of the steel are not significantly better than those in the transverse direction. Subdivi- Maximum core loss AISI type Nominal at 60 Hz and B = sions of these steels include semiprocessed (approximate (Si + AI) Thickness ASTM 1.5 T (15 kG) grades and the fully processed grades. The former equivalent) content, % mm in. designation W/kg W/lb must be given a heat treatment by the purchaser. Fully processed grades are process annealed by Nonoriented the manufacturer. Semiprocessed (ASTM A 683)(a) M-47 1.10 0.64 0.025 64S350 7.71 3.50 Table 8 gives examples of properties specified 1.10 0.47 0.019 47S300 6.61 3.00 by ASTM and American Iron and Steel Institute M-45 1.70 0.64 0.025 64S280 6.17 2.80 (AISI) for standard grades of nonoriented and 1.70 0.47 0.019 47S250 5.51 2.50 oriented electrical steel. The AISI designations M-43 2.00 0.64 0.025 64S260 5.73 2.60 were adopted in 1946 to eliminate the wide vari- 2.00 0.47 0.019 47S230 5.07 2.30 M-36 2.40 0.64 0.025 64S230 5.07 2.30 ety in nomenclature formerly used. When origi- 2.40 0.47 0.019 47S200 4.41 2.00 nally adopted, the AISI designation number ap- M-27 2.70 0.64 0.025 64S213 4.69 2.13 proximated ten times the maximum core loss,* in 2.70 0.47 0.019 47S 188 4.14 1.88 watts per pound, exhibited by 29 gage (0.36 mm, ... 3.00 0.64 0.025 64S194 4.28 1.94 ... 3.00 0.47 0.019 47S178 3.92 1.78 or 0.014 in.) samples when tested at a flux density Fully processed (ASTM A 677)(b) of 1.5 T (15 kG) and a magnetic circuit frequency ... 0.50 0.64 0.025 64F600 13.22 6.00 of 60 Hz. Note that fully processed M-36 tested 0.80 0.47 0.019 47F450 9.92 4.50 as 0.36 mm (0.14 in.) strip now has a maximum M-47 1.05 0.64 0.025 64F470 10.36 4.70 1.05 0.47 0.019 47F380 8.38 3.80 allowable core loss of 4.2 W/kg (1.9 W/lb), not an M-45 1.85 0.64 0.025 64F340 7.49 3.40 approximate level of 7.9 W/kg (3.6 W/lb). Low 1.85 0.47 0.019 47F290 6.39 2.90 M-43 2.35 0.64 0.025 64F270 5.95 2.70 2.35 0.47 0.019 47F230 5.07 2.30 * The term core loss, as applied to electrical steel, is a quantita- M-36 2.65 0.64 0.025 64F240 5.29 2.40 tive measure of the rate at which electrical energy is converted 2.65 0.47 0.019 47F205 4.52 2.05 to thermal energy during 50 or 60 cycle ac magnetization. Core 2.65 0.36 0.014 36F 190 4.19 1.90 loss is separated into two components: hysteresis loss and eddy M-27 2.8 0.64 0.025 64F225 4.96 2.25 current loss. 2.8 0.47 0.019 47F190 4.19 1.90 2.8 0.36 0.014 36F180 3.97 1.80 M-22 3.2 0.64 0.025 64F218 4.80 2.18 1150 3.2 0.47 0.019 47F185 4.08 1.85 3.2 0.36 0.014 36F168 3.70 1.68 M-19 3.3 0.64 0.025 64F208 4.58 2.08 A 1025 - ~440 *C 3.3 0.47 0.019 47F174 3.83 1.74 lID O 3.3 0.36 0.014 36F158 3.48 1.58 M- 15 3.5 0.47 0.019 47F 168 3.70 1.68 900 o 3.5 0.36 0.014 36F145 3.20 1.45 O O Oriented > 775 -1-, Fully processed (ASTM A 876)(c) M-6 3.15 0.35 0.014 35G066 1.45 0.66 650 3.15 0.35 0.014 35H094 2.07(d) 0.94 C M-5 3.15 0.30 0.012 30G058 1.28 0.58 i,,,,

"1" 3.15 0.30 0.012 30H083 1.83(d) 0.83 525 M-4 3.15 0.27 0.011 27G051 1.12 0.51 .... -" 520 *C Core 3.15 0.27 0.011 27H074 1.63(d) 0.74 ... 3.15 0.23 0.009 23G046 1.01 0.46 400 I I I I I ... 3.15 0.23 0.009 23H071 1.56(d) 0.71 0 2 4 6 8 10 12 ... 3.15 0.27 0.011 27P066 1.45(d) 0.66 "lime, h ... 3.15 0.30 0.012 30P070 1.54(d) 0.70

Fig. 8 Surface (case) and core hardness as functions of (a) Refer to ASTM A 683-84 and companion specification A 683M-84 (metric) for detailed information. (b) Refer to ASTM A 677-84 ion nitriding time and temperature for 18Ni (300) and companion specificationA 677M-83 (metric) for detailed information. (c) Refer to ASTM A 876-87 and companion specificationA maraging steel. Source: Ref 20 876M-87 for detailed information. (d) B (magnetic induction) = 1.7 T (17 kG). 774 / Surface Engineering of Irons and Steels

Table 9 Types of core plate coatings used for lowering core losses in electrical steels applied field strength for a 0.26 mm (0.0104 in.) thick silicon steel sheet. Core plate designation Description C-1 An organic enamel or varnish coating sometimesused for cores not immersed in oil. It enhances punchability and is resistant REFERENCES to ordinary operating temperatures. It will not withstand stress-reliefannealing. C-2 An inorganic insulation consisting of a glass-like film formed during the high-temperatureannealing of electrical steel, 1. L.E Pease 111, Ferrous Powder Metallurgy Ma- particularly grain-orientedelectrical steel, as the result of the reaction of an applied coating of MgO and silicates in the surface of the steel. This insulation is intended for air-cooled or oil-immersedcores. It will withstand stress-reliefannealing terials, Properties and Selection: Irons, Steels, and has sufficient interlamination resistance for wound cores of narrow-width strip such as in distribution transformers. It and High-Performance Alloys, Vol 1, ASM is not intended for stamped lamination because it is abrasive to dies. Handbook, ASM Intemational, 1990, p 801- C-3 An enamel or varnish coating intended for air-cooled or oil-immersedcores. C-3 enhances punchability and is resistant to 821 normal operating temperatures. It will not withstand stress-reliefannealing. C-4 Consists of a chemically treated or phosphated surface useful for air-cooled or oil-immersedcores. It will withstand stress- 2. W.B. James, M.J. McDermott, and R.A. Pow- relief annealing in relativelyneutral atmospheres. ell, Powder Forging, Forming and Forging, Vol C-5 An inorganic insulation similar to C-4 but with ceramic fillers (such as colloidal silica) added to increase the eleclrical 14, ASM Handbook (formerly 9th ed., Metals insulation properties. C-5 can be used in air-cooledor oil-immersedcores and will endure stress-reliefannealing. Handbook), ASM Intemational, 1988, p 188- Source: ASTMA 345 211 3. Materials Standards for P/M Structural Parts, MPIF Standard 35, Metal Powder Industries Federation, 1990 0.65 Core insulation must be sufficiently thin and 4. H.E. Boyer, Secondary Operations Performed uniform so as to have no more than 2.0% effect on P/M Parts and Products, Powder Metallurgy, 0.55 on the lamination factor (solidity of the core). To m Vol 7,ASMHandbook (formerly 9th ed., Metals calculate the required insulation for most opera- ~" 0.45 Handbook), ASM Intemational, 1984, p 451- tions at power frequency, the square of the resis- 462 O m tivity, in ohm-centimeters per lamination, should 5. H.A. Ferguson, Heat Treating of Powder Met- L_ 0.35 O at least equal the square of the width of the mag- O allurgy Steels,Heat Treating, Vol 4, ASM Hand- N 0.25 netic path, in inches. This usually ensures a neg- -ir- __~- Laser treated book, 1991, p 229-236 O ligible interlaminar loss that is less than 1.0% of r,D 6. L.E Pease 111, Mechanical Properties of Steam 0.15 the core loss. Blackened P/M Materials, in Modern Develop- Ceramic Films. Japanese electrical steel pro- 0.05 I I I I ments in Powder Metallurgy: Proceedings of 9 11 13 15 17 19 ducers have reported significant improvement in the International Powder Metallurgy Confer- core loss as a result of ion plating of TiN and CrN Test induction, kG ence, Metal Powder Industries Federation, 1988 ceramic coatings (Ref 22). Application of these ce- 7. W. Rembges, Ion Nitriding Applications Grow Fig. 9 Effectof laser scribing on the core loss of a high- ramic f'tlrns on chemically polished grain-oriented for Automotive Components, Heat Treat., permeability grain-oriented electrical steel. electrical steel sheet increased the magnetic flux March 1990 Source: Ref 23 density by 0.004 to 0.015 T (0.04 to 0.15 kG) and 8. A.M. Bayer, Wrought Tool Steels, Properties lowered the core loss by 0.12 to 0.20 W/kg (0.05 to and Selection: Irons, Steels, and High-Perform- 0.09 W/lb). By laser domain refining (see discus- ance Alloys, Vol 1, ASM Handbook (formerly metal laminations that are fabricated by punching sion below), it was possible to further improve the 10th ed., Metals Handbook), ASM Intema- from thin sheets of metal, and which are sub- core loss by 0.04 to 0.09 W/kg (0.02 to 0.04 W/lb). tional, 1990, p 757-779 sequently assembled to form a core. Using this dual ceramic filmAaser treatment, electri- 9. K.E. Pinnow and W. Stasko, P/M Tool Steels, Interlaminar insulation is necessary for high cal steels with original core losses of 0.88 W/kg (0.4 Properties and Selection: Irons, Steels, and electrical efficiency in the magnetic core, W/lb) were improved to an ultralow core loss of High-Performance Alloys, Vol 1, ASM Hand- whether the application is static or rotating. For 0.55 W/kg (0.25 W/lb), which corresponds to an book (formerly 10th ed., Metals Handbook), small cores used in fractional-horsepower mo- improvement of about 40% (Ref 22). ASM Intemational, 1990, p 780-792 tors, an oxide surface on the laminations may Laser Magnetic Domain Refinement. Core 10. J.L. Maloney 111, Friction and Wear of Tool insulate the core adequately. Insulations of AISI loss improvements of 2 to 14% can be achieved by Steels, Friction, Lubrication, and Wear Tech- types C-I, C-2, C-3, C-4, and C-5 are used for rapid scanning (typically 100 m/s, or 325 R/s) a nology, Vol 18, ASM Handbook, ASM Intema- more rigorous requirements. Table 9 describes high-powered focused laser beam across the surface tional, 1992, p 734-740 the characteristics of these various core coatings. (transverse to the rolling direction) of grain-oriented 11. H.C. Fiedler and R.J. Sieraski, Boriding Steels Organic-Type Insulation. Types C-1 and C-3 glass-coated (Si-Mg-P-AI glass) 3% silicon-irons for Wear Resistance, Met. Prog., Feb 1971, p are organic and cannot be successfully applied to. (Ref 23). There is no flame, spark, or smoke gener- 102-103 laminations before annealing. They are unsuitable ated during this process, which is also referred to as 12. O.N. Guy, Boronizing--A Surface Heat Treat- for electrical equipment operated at high tempera- laser scribing, and the material/coating shows no ment for Critical Wear Surfaces, New Develop- tures or for power transformers with certain types of visible surface change. The improvement in core ments in Tool Materials and Applications, coolants. However, they improve the punchability loss is due to a thermal shock imparted to the micro- Blinois Institute of Technology, 1977 of the sheet steel. structure which causes slip plane dislocations to 13. T.D. Deming, Steam Treating Emerges as Im- Inorganic-Type Insulation. Inorganic types form, thereby producing new magnetic domain wall portant Cog in Metal Surface Engineering, In- C-4 and C-5 are used when insulation requirements boundaries (Ref 23). By adjusting the spacing of the dustrial Heating, Jan 1990, p 28-30 are severe and when annealing temperatures up to scanned laser lines, the energy lost due to moving 14. E Wendl, Current Trends in Surface Treatment 790 °C (1450 °F) must be withstood. Typical values domain walls back and forth under the action of the of Tools Used for Plastics Processing, Thyssen of interlaminar resistance for these two types are applied ac field in the transformer is minimized. The Edelstahl Tech. Ber., special issue, May 1990, p between 3 and 100 f~. cm/lamination under a pres- laser lines restrict the length of the domains and also 82-99 sure of 2070 kPa (300 psi). These coatings also can act to control the width of the domains. Thus by 15. H.A. Sundquist, E.H. Sirvio, and M.T. Kurki- be made to impart residual tensile stresses in the adjusting the spacing of the laser lines, the domain nen, Wear of Metalworking Tools Ion Plated steel substrate, which can improve magnetic prop- sizes can be controlled, i.e., refined. Figure 9 illus- "O¢-lthTltanium Nitride, Met. Tech., Vo110, 1983, erties. trates the improvement in core loss as a function of p 130-134 Surface Engineering of Specialty Steels / 775

16. C. Wick, HSS Cutting Tools Gain a Productiv- High-Performance Alloys, Vol 1, ASM Hand- Handbook (formerly 10th ed., Metals Hand- ity Edge, Manufacturing Engineering, May book (formerly 10th ed., Metals Handbook), book), ASM Intemational, 1990, p 761-781 1987, p 38 ASM Intemational, 1990, p 793-800 22. Y. Inokuti, K. Suzuki, and Y. Kobayashi, Grain 17. G.R. Fenske, Ion Implantation, Friction, Lubrica- 20. K. Ozbaysal and O.T. Inal, Surface Hardening Oriented Silicon Steel Sheet With New Ce- tion, and Wear Technology, Vol 18, ASM Hand- of Marage Steels by Ion Nitriding without Re- ramic Films Chamcterize_A by Ultra-Low Iron book, ASM International, 1992, p 850-860 duction in Core Hardness, in Ion Nitriding, Loss, Materials Transactions, J/M, Vo133 (No. 18. M. Hsu and P.A. Molian, Wear, Vol 127, 1988, ASM Intemational, 1986, p 97-115 10), 1992, p 946-952 p 253 21. D.W. Dietrich, Magnetically Soft Materials, 23. G.L. Neiheisel, Laser Magnetic Domain Re- 19. K. Rohrbach and M. Schmidt, Maraging Steels, Properties and Selection: Nonferrous Alloys finement, LIA Vol 44, Proc. Conf. ICALEO Properties and Selection: Irons, Steels, and and Special-Purpose Materials, Vol 2, ASM 1984, p 102-111